Although a popular meme like the platform/publisher duality it isn't a valid concept legally - at least in the US conception. Not any more than being able to get away with specific death threats as "a promise". That sort of "marginal loophole" is frowned upon in the jurisprudence period. The prevailing form of restrictions that have survived are specific and tailored invalidations of forms of speech protection - threats, fraud, and libel.

Not to say that special pleading for abridging rights hasn't embedded itself via ex-nilho doctrines turned precedent.